This invention relates to a gas delivery system for use with plasma treatment apparatus.
In many processes, such as plasma etching, chemical vapour deposition and plasma-enhanced vapour deposition, it is desirable to introduce into a reactor chamber one or more gas or vapour in a manner such that the gas or vapour is relatively evenly distributed across the chamber of the plasma apparatus. This is generally achieved by a device known as a showerhead and typically these comprise a casing defining a plenum for receiving gas from an inlet and an outlet plate defining a large number of fine outlets.
Such showerheads frequently also comprise an electrode and generally have to satisfy a number of requirements and, at times, these are conflicting. Thus they must adequately distribute gas flow generally in a uniform or controlled manner over a large area; they must provide a suitable RF electrode; they must ionize gas that passes through the gas outlets to as high a degree as possible; they must not ionize the gas within the plenum; they must avoid shedding particles upon a workpiece; they must be heat emissive in a uniform or controlled manner over a large area; they must be as unreactive as possible with the process chemistries and they must be thermally and mechanically stable. Although the above features have been set out as being essential, in reality Designers have compromised on many of these features.
Examples of such showerheads are described in U.S. Pat. Nos. 4,854,263 and 5,423,936. It will be noted that the outlet plate or manifold acts as the RF electrode in these cases. As the gas pressure is higher on the gas inlet side rather than the downstream side, it is a frequent problem that plasmas ignite inside the showerhead plenum. This can result in detrimental affects on the processing system due to particals being created and falling on the workpiece and the erosion of parts which are not designed to be exposed to a plasma. There can also be significant manufacturing difficulties arising from the need for a very precise array of holes having very precise small diameters. The exposure of the manifold to the plasma in the chamber necessitates the use of particular materials, which often are not those which are most convenient for machining to form the array of holes in the manifold.
From one aspect the present invention consists in a gas delivery system for plasma treatment apparatus comprising a gas inlet, a plenum, a gas delivery plate, having an array of apertures, extending across the downstream end of the plenum characterised in that the system further comprises a separate ionizer plate adjacent the downstream face of the delivery plate, the ionizer plate being formed with openings that are aligned with respective apertures in the gas delivery plate.
It will be appreciated that in this arrangement the gas delivery plate or manifold is not RF driven in sue although it may be capacitively coupled to the RF, if the gas delivery plate and ionizer plate are in close proximity.
The separate construction of the ionizer plate (or RF electrode) and the gas delivery late or manifold immediately introduces a significant number of advantages:
1. The ionizer plate can be manufactured from a material which is well suited to a plasma environment.
2. The openings in the ionizer plate do not define the gas flow and so they can be larger and more readily machined.
3. The gas delivery plate or manifold no longer has to be manufactured from a xe2x80x9cplasma resistantxe2x80x9d material and a material which is suitable for the requisite machining can be selected.
4. The gas delivery plate or manifold may be earthed if it is D.C. electrically conducting and thus act as a dark space shield or alternatively, as it no longer needs to function as an electrode, it can be made of some other suitable material e.g. a ceramic.
5. The gas delivery plate or manifold and the ionizer plate or electrode may be separately replaceable reducing spare part costs.
6. The gas delivery manifold can be extremely thin, thus increasing the range of manufacturing techniques available for forming the apertures. In this case it will use the structural strength of the ionizer plate and simply rest on the upstream surface of that plate.
7. The downstream face of the ionizer plate can readily be shaped to enhance uniformity or other process characteristics.
8. If a thermal break is provided between the gas delivery plate and the ionizer plate or, if, for example the gas delivery plate is made of ceramic, the ionizer plate can be allowed to become hot, without any concerns about the seals between the gas delivery plate or manifold and the casing defining the plenum. Currently this is a significant problem due to the temperature limits on the elastomeric seals or the material limitations created by clamping arrangements.
It will be understood that the Designer of a gas delivery system in accordance with the invention can utilise one or more of these advantages, depending on the particular apparatus in which the system is to be used.
The openings in the ionizer plate may have a larger cross section than the apertures and that cross section may increase in a downstream direction. As has been mentioned the gas delivery plate may be electrically conducting an earthed or near earthed or alternatively it may be made of electrically non-conducting material.
The system may further comprise a secondary gas delivery plate downstream of the ionizer plate where a process requires a xe2x80x9cremotexe2x80x9d plasma source.
Any or all of the plates may incorporate heating or cooling devices or additionally, or alternatively, a further plate, incorporating a heating or cooling device, may be provided and may be sandwiched with the existing plates.
The invention also includes plasma apertures incorporating a gas delivery system as defined above and further including an RF power supply connected to the ionizer plate.